High deductible health plans (HDHPs) have lower premiums, but require patients to pay the full cost of most medical care until annual deductibles of $1000 to $6000 are met. HDHPs have proliferated rapidly over the last decade. Health Savings Account (HSA)-qualified HDHPs are the most rapidly growing HDHP arrangement, now covering 14% of commercially insured Americans. These plans subject most health care services (including medications) to high annual deductibles but allow pre-tax contributions to accounts that can be used for medical costs. Advocates believe that exposing patients to greater costs will create activated consumers who will seek higher-value health care and adopt healthy behaviors, while critics fear that these economic barriers may reduce access to needed care. Our ongoing project in the Natural Experiments in Diabetes Translation (NEXT-D) initiative (5U58DP002719) has determined that diabetes patients with HSA-HDHPs experience the largest reductions in hospital access and adherence to key medications for chronic illness. Although these findings are concerning, federal regulations allow employers to tailor HSA-HDHPs through two mechanisms that could improve access to care: (1) Exempting certain chronic medications from deductible requirements (i.e., medications cost $0 to patients) through preventive drug lists ($0-PDs), and (2) Generously funding HSAs so that employees pay less out-of-pocket. Thus, HSA-HDHPs represent both a risk to diabetes patients but also an opportunity for employers to improve care through tailored cost sharing reductions. In this application, we will examine effects of these two important variations in HDHP design on diabetes secondary prevention, high priority care, diabetes complications, and costs, both overall and among vulnerable subgroups (men, low socioeconomic status, Black or Hispanic, severe diabetes, high comorbidity). We will employ a cutting-edge interrupted time series study design that approximates the estimates of randomized controlled trials. We will draw from a 12-year rolling cohort (2005-2017) of >60 million members insured by a large national health plan, with 30,000 diabetes patients who experience employer-mandated transitions to HSA-HDHPs. We will also conduct economic analyses from the employer and member perspectives of the impacts of $0-PDs and generously funded HSAs on total health care and out-of-pocket costs. Our findings will allow policy makers and employers to develop tailored, next-generation health insurance designs that optimize quality and equity for diabetes patients who are most vulnerable under HDHPs.